Field of the Invention
The invention generally relates to a solid-state relay. In particular, the invention relates to a digitally controlled programmable solid-state relay which may be used in railway applications. More particularly, the invention relates to a programmable solid-state relay, which may be used in retro-fitting an existing electromechanical relay. It will be convenient to hereinafter describe the invention in relation to this particular application. It should be appreciated however that the present invention is not limited to that application only.
Description of Related Art
Relays are used extensively in the railway industry for the propagation of electrical signals through the railway signalling system and time delay relays are similarly used to delay the propagation of those electrical signals for a pre-determined period of time.
Such time delay relays commonly in use by railway systems around the world are generally based upon a resistor-capacitor circuit (“RC circuit”) time delay. A storage capacitor is charged to a pre-set level and then discharged through a resistor.
In an RC circuit, the value of the time constant in seconds is equal to the product of the circuit resistance in Ohms and the circuit capacitance in Farads, i.e. τ=R×C. τ is the time required to charge the capacitor, through the resistor, to 63.2% of full charge or to discharge it to 36.8% of its initial voltage.
Various delay circuits are known in the art. One approach by Ma, as disclosed in U.S. Pat. No. 7,961,030, uses delay circuits that include a resistor and a capacitor in series. The time delay is related to the resistance of the resistor and the capacitance of the capacitor.
In another approach by Darrow, which is disclosed in U.S. Pat. No. 4,044,272, a fail-safe time delay circuit for providing a time interval is similarly described. The time delay circuit includes a resistance-capacitance charging network, which is connected to a direct current (“DC”) supply source by a switching device. The potential charge developed on the capacitor powers an inverter to produce alternating current (“AC”) signals having a given frequency. The AC signal is then fed to a multi-stage tuned amplifier, having a resonant circuit tuned to the given frequency. The amplified AC signals are applied to a voltage doubling network, which normally energizes a load and which maintains the load energized for no longer than the definite time interval after the opening of the switching device.
Hayden, in U.S. Pat. No. 4,276,483, describes a timed switch utilizing a resistive capacitor relaxation oscillator. However, a drawback of this technique is that resistor and capacitor values are nominal only, which prevent an accurate prediction of the resultant time delay and circuits often require fine adjustments to achieve the desired time delay.
It has proved to be problematic in the art to develop practical RC circuits, which provide accurate and predictable timing, because the rate of current discharge from the capacitor is exponential rather than linear with time. Time delay relays typically utilize a fixed value capacitor and a variable resistor or potentiometer to select the desired delay period.
In practice, setting the time delay is usually one of trial and error. The methodology followed is to first set the potentiometer at some nominal value. The relay is then energized and the delay time is measured. The potentiometer is then adjusted, the relay reset and the delay time measured again. This process is repeated until the desired time delay is achieved or approximated.
A further drawback of the RC timing technique is that the values of these discrete components can be affected by both temperature and ageing.
One advantage offered by the present invention is that it does not rely upon the charge/decay rate of a timing capacitor to control the delay time. Further, the level of complexity, which was previously mandatory, has been substantially simplified.
Schofield, in U.S. Pat. No. 4,351,014 describes a fail-safe solid-state relay for AC devices, which employs triodes for alternating current (“TRIACs”). This approach cannot be applied to DC devices as the central component (“TRIAC”) is limited to AC operation.
Koga et al., in U.S. Pat. No. 4,855,612, also provides a relay, which is operable to delay the transition of a plurality of switches using a capacitor as the timing means.
Existing electromechanical relays, such as those used by British Rail, for example, often experience a number of problems, such as high contact resistance, mechanical wear and tear, susceptibility to environmental conditions, variability of performance based on mechanical and material variation, for example, of the contact spring tension and the like.
In addition, non-time delay relays suffer from delays caused by their design. The present invention incorporates the use of a transistor or some other solid-state-based switching circuit instead of the mechanical-type contact. Further, the present inventors have surprisingly found that by retrofitting the present time delay relay to circuits employing existing electromechanical relays, the function of existing relays can be substantially emulated. In this way, the relay of the present invention offers the ability to detect component failures and thereby substantially prevent unsafe switch states from occurring.
A further issue is that there are occasionally temporary power interruptions, during which the voltage level supplied to the relay from an external power source is interrupted. This drop in voltage can sometimes cause a problem with the reliability of the relay.
The present invention seeks to overcome, or at least substantially ameliorate, at least some of the disadvantages and shortcomings of the prior art.